A reflecting telescope (reflector) is an optical telescope which uses a combination of curved or plane (flat) mirrors to reflect light and form an image (catoptric), rather than lenses to refract or bend light to form an image (dioptric).
A curved primary mirror is the reflector telescope's basic optical element which creates an image at the focal plane. The distance from the mirror to the focal plane is called the focal length. Film or a digital sensor may be located here to record the image, or an eyepiece for visual observation.
Mirrors eliminate chromatic aberration but still produce other types of aberrations. In general, on axis they produce spherical aberration—the outer and inner zones of the telescope do not share a common focus. This was the construction flaw in the Hubble Space Telescope mirrors. Spherical aberration can be eliminated with aspheric (non-spherical) mirrors. Off axis, additional aberrations will become apparent.
Nearly all large research-grade astronomical telescopes are reflectors. Lenses work because of the phenomenon of refraction rather than reflection. Therefore, in a lens, the entire volume of material has to be free of imperfection and nonhomogeneities, whereas in a mirror, only one surface has to be perfectly polished. Refraction of light is uniform only across a single wavelength. Light of different wavelengths travels through any translucent medium other than a vacuum at different speeds. Thus, chromatic aberration, the focusing of light of different wavelengths occurring at different focal points, occurs in uncorrected lenses, causing the creation of an aberration-free large lens to be a costly process. Because a mirror reflects different wavelengths at the same angle, chromatic aberration is not a concern.
Reflectors work in a wider spectrum of light since certain wavelengths are absorbed when passing through glass elements like those found in a refractor or catadioptric. Collection and transmission of the spectrum of light is an important role of a telescope, and thus absorption of some portion of the spectrum compromises the purpose of the telescope. People often mistakenly believe that the power of a telescope lies in its ability to magnify objects. Telescopes actually work by collecting more light than the human eye can capture on its own. The larger a telescope's mirror, the more light it can collect, and the better its vision.
Reflecting mirrors are superior to lenses in their ability to resist deformation due to the effects of gravity upon their structure. Mirrors can be supported from behind, and they do not absorb any wavelengths of light, or cause chromatic errors, the way lenses do. Larger mirrors, however, require an elaborately complex structural support system to keep the structure of the mirror from collapsing under its own enormous weight. Also, the larger a surface of a mirror, the thicker it must be in order to withstand gravitational affects that could alter its shape. As the size swells, therefore, the cost of the mirror becomes exorbitant.
An additional issue with weight is the cost of sending such weight into space. Propellant will be more than 85% of the mass that needs to be lofted into Low Earth Orbit (LEO) in placement of any object. Thus, every gram of weight on earth requires an additional nearly 6 grams of propellant to place that gram into LEO. Thus, when the Hubble Space Telescope's launch in 1990 placed a reflective mirror in an orbit about Earth, the size of the objective mirror was limited in a “weight versus cost” decision. Yet, Hubble's position above the atmosphere, which distorts and blocks the light that reaches our planet, gives it a view of the universe that typically far surpasses that of ground-based telescopes. Weight is an ongoing concern as more space telescopes are planned.
One solution to the problem that mass and its gravitational attribute, weight, have on mirrors has been addressed by 36 hexagonal segments of the 400 inch Keck Telescope at the W. M. Keck Observatory at the summit of Mauna Kea in Hawaii. The Keck Telescope's revolutionary design employs 36 individual lightweight glass mirror segments which together, under the control of a computer, maintain a single, precise hyperbolic surface accurate to within a millionth of an inch. They are not 36 separate hyperbolic mirrors. They are 36 segments of a single hyperbolic mirror. The attitude of each of the 36 mirror segments is adjusted twice a second under the control of a computer. The computer looks at input provided by sensors located at each segment's edge. The computer drives three actuators underneath each segment to keep all 36 segments in a perfect hyperbolic shape as the telescope moves, or as it is buffeted by the wind. Thus, with the availability of computer control, it was not necessary to create a single, rigid, monolithic 10-m diameter piece of glass, which would be very difficult or impossible to deal with.
However, the Keck design still suffers from a vast shortcoming. Both telescopes are primarily optical telescopes with coatings that allow no ability to explore either of the near infrared or ultraviolet extremes of the mirrored spectrum. An optical coating is a thin layer of material placed on an optical component such as a lens or mirror which alters the way in which the optic reflects and transmits light. One type of optical coating is the high-reflector coating which can be used to produce mirrors which reflect much of the spectrum of light which falls on them. More complex optical coatings exhibit high reflection over some range of wavelengths, and anti-reflection over another range, allowing the production of dichroic thin-film optical filters. All coatings suffer from the fact that a coating can only be optimized to reflect a portion of the spectrum.
Additionally, different types of mirrors exist. The familiar metallic mirror is omnidirectional, which means it reflects light from every angle. It also absorbs a significant portion of the incident light.
Dielectric mirrors, unlike metallic mirrors, do not conduct electricity and therefore can reflect light more efficiently. Light travels in dielectric materials at speeds that are lower than in air. When light traveling in a particular direction through one type of dielectric material encounters another type, part of the light is reflected while the other part is transmitted at a different angle.
Dielectric mirrors are made of multiple layers of transparent dielectric materials. Such materials, which can be made to be extremely low loss compared to their metallic counterparts, are used to reflect a prescribed range of frequencies coming from within a limited set of angles. Dielectric mirrors are used in devices such as lasers, which need very high reflectivity.
What is missing in the art is a means for using a single telescope structure to support a greater primary mirror element and further to allow for the observation of phenomena at different spectral ranges within the single telescope.